The present invention concerns the manufacture of a selective, stable and sulfur resistant palladium-gold catalyst, as well as its use in the selective hydrogenation of diolefinic and/or acetylenic hydrocarbons.
The processes for the conversion of hydrocarbons at high temperatures, such for example as steam cracking, visbreaking, coking and catalytic cracking, produce a large amount of gaseous and liquid olefinic and diolefinic unsaturated hydrocarbons such as, for example, ethylene, propylene, butadiene, butenes as well as hydrocarbons boiling in the range of gasolines and having also a high content of diolefinic and olefinic components.
The gaseous olefinic and diolefinic hydrocarbons with two to four carbon atoms obtained by these processes also contain a certain amount of acetylenic hydrocarbons. The content in these hydrocarbons varies in accordance with the severity of the conversion treatment but is always too low for making it reasonably possible to separate and use them in, for example, petrochemical processes. However, their presence in addition to olefinic and diolefinic hydrocarbons makes impossible to upgrade the latter. Accordingly, only their use as mere fuel could be contemplated and this use is more and more ruled out for obvious economical reasons.
There is accordingly an attempt to use light olefinic hydrocarbons either as petrochemical bases or as fuel components.
Thus, for example, propylene obtained from catalytic cracking may be used as monomer for producing polypropylene or as reactant in the production of dimers forming an excellent fuel base.
Thus, in a general manner, the upgradings in petrochemistry or as fuel make necessary a selective hydrogenation treatment so as to selectively convert the most unsaturated impurities such, for example, as the acetylenics in the olefinic and diolefinic cuts or the diolefins in the olefinic cuts.
However, the new developments, mainly in connection with the oil crisis, pose new problems in the field of hydrogenation which cannot be satisfactorily solved with the present catalysts.
A first problem is connected with the development of the processes for converting heavy oils at high temperatures, such as visbreaking, coking or catalytic cracking, which generate olefinic cuts of higher content in heteroatomic impurities such as sulfur compounds. Whereas the light cuts (C.sub.2, C.sub.3, C.sub.4) issued from these processes were previously used essentially as fuel, it is now desired to better upgrade them by converting them, either to basic products for chemistry, or to a fuel base.
This requires the use of selective hydrogenation treatments. However, the presence of sulfur-containing impurities results in a progressive poisoning of the most currently used catalysts such as supported palladium and makes it necessary either to considerably increase the catalyst amount or to proceed to numerous regenerations of high power cost.
A second problem, also connected with the requirement of better upgrading the oil cuts also appeared. Thus, the necessity to increase the yields to olefins has led to an increase of the operating temperatures of the steam-cracking furnaces; this has led in the various cuts so produced, to a higher content of acetylenic and diolefinic compounds. In particular, the raw steam-cracking C.sub.4 cut contains, in addition to butadiene and butenes, a relatively high proportion of vinylacetylene (1 to 2% by weight). This results, when effecting the extractive distillation of butadiene, in a substantial loss of said compound which is used to dilute the acetylenic concentrate issuing from the bottom of the column. This dilution is made necessary for security reasons and this cut which, unhappily, cannot be subjected to a proper chemical upgrading, is generally burned at the present time.
In order to decrease said butadiene loss, it is thus desirable to previously hydrogenate vinylacetylene so as to lower its content as much as possible while tolerating only the smallest possible butadiene loss. The conventional catalysts with a palladium base provide, under certain conditions, for butadiene yields of 99% or more with respect to the butadiene input in the hydrogenation reactor.
However, it is observed that this type of catalyst deactivates relatively quickly as a result of a progressive leaching of palladium by the charge; this metal leaching is the higher as the vinylacetylene content of the charge is higher.
The third problem encountered is also connected with the increase of the acetylenic hydrocarbons concentration in the light steam-cracking cuts. As a matter of fact, with the conventionally used palladium catalyst, an oligomer formation, parallel to the hydrogenation, has been observed, said formation being the more substantial as the acetylenics content to hydrogenate is higher. This parasitic reaction had a double disadvantage, the first being to lower the yield to the desired olefinic product (case of propylene for example), the second being a progessive clogging of the catalyst which requires frequent regenerations (case of gaseous phase hydrogenation of C.sub.2).
To summarize, the usual palladium catalysts suffer from three main disadvantages:
the first resulting from their too high sensitivity to poisoning by sulfur compounds, PA1 the second being the progressive dissolution of palladium in the C.sub.4 cuts containing vinylacetylene, PA1 the third resulting from the too substantial formation of oligomers.
The use, for the selective hydrogenation, of catalysts containing palladium and gold has been proposed (FR No. 2 482 953, GB No. 802 100). Gold is introduced as chloroauric acid. These catalysts are more active and more selective than catalysts containing palladium alone, but palladium tends to be quickly removed from the catalyst by the acetylenic hydrocarbons such as vinylacetylene. In addition, the oligomer formation is substantial. Catalysts may be prepared by simultaneously introducing palladium and gold in a carrier and by taking care to avoid the presence of chlorine in the final catalyst (U.S. Pat. Nos. 3,974,102 and 4,136,062); the activity of these catalysts is not satisfactory.
The object of the invention is the preparation of a catalyst having simultaneously a high resistance to poisoning by sulfur, a high stability with respect to elution by vinylacetylene, as well as a low tendency to oligomers formation.